1. Technical Field
The present disclosure generally relates to determination of a downhole formation fluid property and in particular to the determination of the bubble point pressure for a downhole formation fluid.
2. Background Information
Oil and gas wells have been drilled at depths ranging from a few thousand feet to as deep as five miles. Information about the subterranean formations traversed by the well borehole can be obtained by various techniques. Techniques used to obtain formation information include obtaining one or more core samples of the subterranean formations and obtaining fluid samples produced from the subterranean formations. Fluid samples are often retrieved from the borehole and tested in a rig-site or remote laboratory to determine properties of the fluid sample. Modern fluid sampling includes various downhole tests and sometimes fluid samples are retrieved for surface laboratory testing.
Properties of downhole formation fluids are of great importance to engineers. One important property in particular is the bubble point pressure of the downhole formation fluid. The bubble point pressure is the pressure at which gas evolves from the formation fluid, as the pressure of the formation fluid is decreased. The bubble point pressure is also a point to separate the status of the formation fluid into a saturated state and an undersaturated state. The formation fluid is at the saturated state where there is free gas in the formation fluid, otherwise the formation fluid is at the undersaturated state. To optimize the oil production, the pressure of a reservoir needs to be kept above the bubble point pressure. If the pressure drops below the bubble point during production, gas bubbles will form within the reservoir rock, which will decrease the relative permeability of the oil phase. Also, to precisely determine the bubble point pressure at the surface, the pressure of the formation fluid needs to be kept above the bubble point pressure, as it is transported to the surface.
Present methods used to determine the bubble point pressure of a formation fluid trap a volume of a formation fluid and gradually expand the volume and monitor how the pressure drops. Initially the pressure drops linearly with the expanding fluid volume. The estimated bubble point pressure is the pressure at which the pressure is no longer linearly related to the volume. In practice the current methods for estimating bubble point pressure are not as accurate and reliable as desired, because the formation fluid sampling test data can have uncontrollable noises and the formation sampling fluid may be contaminated. There is a need, therefore, for a method for more accurate estimation the bubble point pressure of a formation fluid.